Changes during the menstrual cycle in the physical properties or chemical content of cervical mucus, are the bases of ovulation detection methods for natural family planning. These methods attempt to determine the fertile period during which the couple abstains from sexual intercourse. Many such methods have been proposed and tested but a simple, consistently reliable method, giving a 3-5 day forewarning of the impeding ovulation, has not been found. Our studies with human endometrium have revealed that guaiacol peroxidase (G-Px) is located maximally in endocervical epithelium and decreases around the time of ovulation. Subsequent studies of cervical mucus G-Px showed that G-Px declined dramatically 3-5 days prior to ovulation and 1-2 days thereafter returned to its high preovulatory levels. G-Px is extracted from cervical mucus with a Ca++ containing buffer and in the presence of H2O2 catalyzes the oxidation of an artificial substrate, guaiacol; the products of this reaction are orange-brown. We propose to study, during the menstrual cycle, daily levels of G-Px activity in the cervical mucus from paid volunteers and compare them with G-Px levels in their vaginal secretions and saliva. Daily plasma levels of LH, Fsh, estradiol and progesterone will document the time of ovulation. Attempts to optimize the G-Px assay, characterize this peroxidase and measure catecholestrogens in these body fluids will be made to improve the method and, possibly, understand its regulation. Our goal is to determine the reliability of G-Px in predicting ovulation in hormonally studied cycles. These results, we hope, will be the basis for a self-administered, simple, dependable test for natural family planning using G-Px measurements in the best of the above three kinds of body function may provide data to develop new methods for improving or retarding reproductive performance.